Baking displacement map with Cycles Experimental
Baking Displacement maps is very easy in Blender Internal. Switch to it.Then unwrap low poly object. Switch baking pass to Displacement, set appropriate distance.Enable Selected to Active. Select low poly object, switch to Edit mode.In UV/Image editor create new image (switch 32bit float on when generating), make sure, that UV map of the object is displayed on this new image.Tab out of edit mode, select first high poly object, then low poly object and hit Bake. Like this: